stages: add org.osbuild.erofs

Erofs is "a lightweight read-only file system"[1]. Imagine squashfs, but with
faster reads.

This commit adds support for creating it. The new stage is heavily inspired by
the squashfs one. I've decided to add all features of mkfs.erofs that looked
useful: All compression types and most of extended options (excluding the
compatibility ones, we can always add them later).

[1]: https://en.wikipedia.org/wiki/EROFS

stages/org.osbuild.erofs

@@ -0,0 +1,98 @@
+#!/usr/bin/python3
+"""
+Create a file containing an errofs filesystem named `filename`.
+
+Buildhost commands used: `mkfs.erofs`
+"""
+
+import os
+import subprocess
+import sys
+
+import osbuild.api
+
+SCHEMA_2 = """
+"options": {
+  "additionalProperties": false,
+  "required": ["filename"],
+  "properties": {
+    "filename": {
+      "description": "Filename for the output",
+      "type": "string"
+    },
+    "compression": {
+      "type": "object",
+      "additionalProperties": false,
+      "required": ["method"],
+      "properties": {
+        "method": {
+          "description": "Compression method",
+          "enum": ["lz4", "lz4hc", "lzma"]
+        },
+        "level": {
+          "description": "Compression level. Note that different methods support different levels. See mkfs.erofs(1) for more details",
+          "type": "number"
+        }
+      }
+    },
+    "options": {
+      "description": "Extended options for the filesystem, see mkfs.erofs(1)",
+      "type": "array",
+      "minItems": 1,
+      "items:": {
+        "enum": [
+          "all-fragments",
+          "dedupe",
+          "force-inode-compact",
+          "force-inode-extended",
+          "force-inode-blockmap",
+          "force-chunk-indexes",
+          "fragments",
+          "noinline_data",
+          "ztailpacking"
+        ]
+      }
+    }
+  }
+},
+"inputs": {
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["tree"],
+  "properties": {
+    "tree": {
+      "type": "object",
+      "additionalProperties": true
+    }
+  }
+}
+"""
+
+
+def main(inputs, output_dir, options):
+    source = inputs["tree"]["path"]
+    filename = options["filename"].lstrip("/")
+    compression = options.get("compression")
+
+    target = os.path.join(output_dir, filename)
+
+    cmd = ["mkfs.erofs", target, source]
+
+    if compression:
+        method = compression["method"]
+        if compression.get("level"):
+            method += f",{compression['level']}"
+        cmd += ["-z", method]
+
+    if options:
+        cmd += ["-E", ",".join(options)]
+
+    subprocess.run(cmd, check=True)
+
+    return 0
+
+
+if __name__ == '__main__':
+    args = osbuild.api.arguments()
+    r = main(args["inputs"], args["tree"], args["options"])
+    sys.exit(r)